If you ask yourself which voltage the fan connectors of the BIGTREETECH E3EZ* board have, you are not alone. The User Manual of the BIGTREETECH E3EZ Board just features a “Mainboard Supply Voltage” description but not special “Fan Voltage”. And this is on purpose - basically the fan features the same voltage as DC_IN. So if your Mainboard is connected to 12V, the fans also should be 12V fans. And likewise with 24V (which is a bit more common).